This invention pertains to the art of metal rolling. More particularly, it relates to the slitting of metal strip of predetermined width and thickness as the last step or as an intermediate step in one or more rolling operations.
Metal which has been rolled to a given thickness in a rolling mill must often be sized to the desired width by a final slitting operation to obtain a finished product of the proper dimensions. The conventional slitting operation, however, introduces a number of problems and undesirable results. It would be preferable (if possible) to slit the metal strip along closely adjacent to one edge to thereby reduce waste by minimizing the amount of trimmed metal which must be recycled. In other words, the portion of the strip which is trimmed off must be recycled by being remelted and again passed through the rolling mill. Such recycling of metal causes a wastage of time, labor and energy. In a typical conventional installation several percent of the throughput of the strip mill must be recycled, which recycled percentage increases with narrowing strip width. This recycling means that the actual useful output at best is only ninety-odd percent of the mill capacity.
However, the slitting operation constitutes cold working of the metal in the localized region bordering upon the slit edge, thereby changing its internal structure and physical properties along the slit edge. The slitting imparts a squeezing or compressive working effect bordering closely to the slit edge. As a result of these non-uniform effects, the physical properties become different along and adjacent to the slit edge of the strip as compared with its remaining cross sectional area, which remains essentially undisturbed by a conventional slitting operation. This conventional slitting of only one edge of a metal strip results in a condition known as "camber," a tendency of the metal strip to assume a sideways or lateral curvature upon leaving the slitter. The strip having camber curves sideways in a direction away from its slit edge, that is, the slit edge is convex as seen looking longitudinally along the strip.
In order to avoid the problem of camber, it is customary to trim metal strip to its desired finished width by simultaneously slitting along both edges. This dual edge trimming has the effect of increasing the quantity of waste metal which must be recycled. In addition to the problem of camber, conventional trimming also results in poor edge profile, edge roughness, and edge sharpness or burr. One of these latter problems is corrected by the trimming of two edges rather than one. When both edges are simultaneously trimmed, the problem of camber may be overcome, but both of the trimmed edges are now likely to exhibit these undesirable characteristics of poor edge quality. They often require subsequent polishing or burr removal treatment.
Finally, it is to be noted that a dual edge trimmed strip is non-uniform. The regions bordering closely to both slit edges are work hardened as compared with the main body of the metal in the strip.
Still another problem created by the use of a conventional slitter is that take-up rolls are required to pull the strip through slitting knives. Therefore, in addition to the number of rolling stands in the mill itself, there must also be a slitting line which usually includes pay-off mechanism feeding the strip into slitting knives with a take-up (or pulling) arrangement downstream from the slitting knives. This need for a separate slitting line increases the overall complexity and cost of the installation and requires additional floor space. Finally, for higher quality production, there is the edge polishing, burr removal, or other equipment for upgrading the rough, sharp edges to be of acceptable quality.
Another problem of conventional slitting is that the material to be slit is required to have sufficient yield strength to withstand any need for being pulled through the slitter knives under tension. Otherwise, strip of low yield strength may become elongated and stretched and thereby lose dimensional tolerance in the resulting slit strips.